1. Field of the Invention
The present invention relates to a novel method for producing an elongated sintered article, particularly relates to a method for producing elongated sintered articles in a form of wire, strand, tape, sheet, rod or the like from powder materials.
The method of the present invention is advantageously applicable to production of wire or rod of ceramics, particularly so called new ceramics or fine ceramics, sintered alloys or their combination, which are difficult of shaping or moulding by conventional process such as wire-drawing, rolling or extrusion of powder material and are difficult of machining or processing after the powder material is sintered.
2. Description of the Related Art
Since new ceramics or fine ceramics and sintered alloys show a wide variety of functions, their application is spreading over a variety of industrial fields. These new or fine ceramics and sintered alloys are utilized in machine parts due to their higher hardness, heat-resistance and dimensional stability, in optical parts due to their high transparency or unique electromagnetic functions, in biological or chemical field such as artificial bone or catalyst because of their superior histocompatibility or chemical-resistance, in electronics or electric parts owing to their improved or unique properties such as electroconductivity or insulative property, and so on.
The term of ceramics and sintered alloys imply any sintered article made by sintering process of inorganic powder material and/or metal powder material. Ceramics are classified into two types of oxide-type and nonoxide type. The oxide type ceramics are divided into simple oxides such as alumina (Al.sub.2 O.sub.3), zirconia (ZrO.sub.2), beryllia (BeO) etc. and compound oxides such as ferrites (MO.Fe.sub.2 O.sub.3), PLZT ((Pb,La) (Zr,Ti)O.sub.3) or the like. In the nonoxide type ceramics, we can mention a variety of compounds such as nitrides such as Si.sub.3 N.sub.4, AlN, carbide such as SiC, WC, boron carbide (B.sub.4 C), partially stabilized zirconia etc. Sintered alloys such as carbide precipitating type cobalt-based alloy are made mainly of tansition metals but are also called as a kind of ceramics.
Ceramics such as tungstencarbide and so called cemented carbides in which carbides of transition metals are bonded with a binder metal such as cobalt show higher hardness and superior tenacity and hence are widely used in a variety of applications such as cutting tools, abrasion resisting parts or the like. It has become more popular to use twistdrills made of this ceramics in the field of the cutting tool and printer head rods for dot-matriprinters as a abrasion resisting part.
Ceramics such as tungstencarbide, alumina or the like posses extremely higher hardness and higher abrasion resistance and are attracting wide public interest. They are used widely in a variety of industries as a ceramic shaft, a ceramic reed or the like.
Ceramics such as siliconcarbide, alumina or the like posses higher strength at high temperatures and improved abrasion resistance and hence are widely used in a variety of applications such as parts which are used at a higher temperature range. Particularly, demand for ceramic parts having an elongated rod-like shape is much increasing in the field of a shaft for engine parts, conveyer rollers which are used at high temperatures, electrodes for automatic welding machines or the like.
Recently, alumina is widely used in the field of electronics as packaging material or substrates or the like.
Carbide precipitating reinforced type cobalt-based alloy consists of 20 to 35 wt % of Cr, 3 to 20 wt % of W, 0.5 to 3 wt % of C and balance of Co and have superior heat-resistance, corrosion-resistance and abrasion-resistance and hence is usually used under high-temperature or corrosive atmosphere. Recently, the carbide precipitating reinforced type cobalt-based alloy has been used in the field of shafts of jet-engines, electrodes for welder, jet-spraying rod etc. and hence demand for elongated wire or elongated rods (hereinafter referred as fine wire) is increasing.
Although ceramics or sintered alloys are used in every industrial field, machining or processing of the sintered body is very difficult to perform because one of their important properties is their harness. That is, sintered body can not be machined or shaped by ordinary cutting tools and so they have been machined only by electro spark machining or diamond cutting techniques. Still more, powder material for these ceramics and sintered alloys are difficult to be shaped or moulded into an elongated article by conventional techniques such as extrusion, rolling or wire-drawing.
Therefore, heretofore, when an elongated article is produced, the powder material is moulded or shaped into an elongated configuration and then sintered in order to minimize after-treatment or after-processing of the sintered article. In this method, it is difficult to produce an elongated article of high quality.
In another technique for producing a rod such as a shaft, a block or billet is preformed by a press and then is machined by cutting into the final shape. This process, however, has not high productivity as well as produce very large amount of loss of expensive powder material. In this process, it is also impossible to produce a long wire or rod because the ratio of longitudinal dimension to cross sectional dimension can not be increased and continuous operation can not be adopted.
In still another prior art, a mixture of powder material and binder of organic compound is extruded into an elongated article or coated onto a supporting belt, and after the organic binder is eliminated in a preliminary sintering stage, final sintering is carried out. It is very difficult in practice to remove the organic binder completely during the preliminary or intermediate sintering step because a large volume of organic binder is added to the powder material and hence a part of carbon remain in the final sintered product, resulting in cause of defects such as cavities or voids which will lower the strength of the final sintered products or deteriorate characteristics of the sintered product whose contents of carbon must be controlled precisely. Still more, it takes long time such as several hours to perform the preliminary sintering for removal of carbon, resulting in lowering productivity.
Therefore, it has been very difficult to produce an elongated article of high quality from powder materials for ceramics.
In case of carbide precipitating type cobalt-based alloy, there are several other techniques which can be applicable to produce an elongated article such as (1) centrifugal Casting, (2) Rotary Spinning in water and (3) metal plating. However,
(1) Although centrifugal casting process is one of rather easier techniques, it is difficult to produce elongated fine articles. The maximum length of the fine wire produced by this technique was about 20 to 30 cm in case of a diameter of 2 mm. There is problem of defects such as cavity which is apt to be produced at the center of the fine wire and microsegregation due to casting operation, resulting in that it is difficult to produce a fine wire having a high quality in strength.
(2) The rotary spinning in water can produce fine elongated wire products but it is very difficult to adjust or control its diameter. Still more, the diameter of the wire is limited to less than 1 mm.
(3) In the metal plating, a carbon fiber is coated with a plating layer of Co, W, Cr or the like which is then diffused to produce an alloy. However, the plating of W is extremely difficult to practice and productivity is also low.
Therefore, there still remain problems to be solved in this field of technology. Particularly, a novel process which is practicable in the field of wire manufacturing or rod manufacturing is strongly requested.
Accordingly, an object of the present invention is to provide a novel method for producing an elongated sintered article of higher quality with the higher productivity and with reduced loss of expensive powder material.